U3 version of Portable Thunderbird 2.0a1?

Nope, there's no plan to do any of the pre-release or experimental (read: more geeky for us geek-type folks) stuff for U3. There are a few reasons for this:

1. U3 is targeted more at general consumers, so it's not a great fit.
2. Most folks that want to test out the new releases of FF and TB don't have U3 drives. Keep in mind that current downloads of the portable apps are 20x that of the U3 ones and the experimental/pre-release stuff like alpha builds is only for a very small subset of those portable users. (As an example... Thunderbird Portable 1.5.0.5 has 14,000 downloads. In comparison, Thunderbird Portable 2.0a1... released the same day... has only 1,200. By straight stats, a U3 release would see about 60 downloads... though that would probably be less since most U3 users don't do pre-release software.)
3. Lots of folks that want to test out the new version of Thunderbird actually use Portable Thunderbird from their local PC. This is because, unlike the standard Thunderbird 2.0a1 installer, it can be installed alongside your Thunderbird 1.5 instance without affecting it. A U3 version couldn't do this.
4. Portable Thunderbird 2.0a1 can co-exist on a portable drive with Portable Thunderbird 1.5 or Thunderbird for U3 1.5. If I did a U3 version of TB 2.0a1 it would have to replace TB for U3 1.5... or I'd have to create another GUID just for the test version.
5. The U3 apps include proprietary (read: closed source) components, so I can't host the downloads (SourceForge is for open source only). Software Central doesn't host alpha, beta or experimental software so that's out, too.
6. It's yet another version to maintain... taking time away from all the other stuff... and there's really no need for that.

Just grab Thunderbird Portable 2.0a1 and run it from your U3 drive. You shouldn't be using it as your everyday email client anyway (it's alpha for a reason).